1. Field of the Disclosure
The present disclosure relates generally to an optical scanning system in an imaging apparatus, and particularly to an over-filled scanner (OFS) scanning system.
2. Description of the Related Art
In various imaging devices which utilize light to form images, optical scanning systems are typically incorporated to scan light beams from one or more light sources onto a target image plane surface. In an electrophotographic imaging device, for example, the image plane surface is typically a photosensitive member. Generally, light beams are swept across the image plane surface by a scanning mirror to form light spots upon the image plane surface along a scan line direction. Commonly used scanning mirrors include rotating polygon mirrors which scan light beams in one direction.
A polygon mirror can have either an under-filled or over-filled facet design. In an under-filled design, the facet length is significantly wider than the incident light beam width such that the beam footprint on a facet never crosses over the edges of the facet from start to end of a scan line operation. On the other hand, an over-filled design has a facet length that is sized such that the incident light beam completely fills a facet over the duration of a scan line operation. In this case, the width of the laser beam after it is reflected by the polygon mirror is determined by the size of the polygon facet.
Generally, in order to have a decent optical performance particularly on laser spot size, width of a light beam striking a polygon facet must be at least some requisite value, such as 4 mm. By comparison, for a given number of polygon facets, the under-filled design would require a larger polygon diameter since size of a facet would have to be wider than the requisite beam width, while the over-filled design would require a smaller polygon diameter since length of a facet only needs to be at least the same as the requisite beam width. Thus, scanning systems that employ polygon mirrors with larger number of facets can be implemented at lower costs using the overfilled design. In addition, polygon mirrors having smaller diameters are not only significantly less expensive, but also run faster, have less acoustic noise and contamination on the polygon facets.
One of the challenges in the over-filled facet design is to achieve a sufficiently wide incoming beam with relatively small wavefront error for a good quality beam. In some existing approaches, beam expanding optic sets have been used to expand laser beams along a scan direction. As an example, FIG. 1 illustrates a schematic layout of a scanning system 10 employing the over-filled polygon facet design. Generally, light beam 15 from a light source 20 is collimated or slightly converged after passing through a collimation lens 25, and a prescan lens 30 is used to minimize pyramidal angle error of a polygon mirror 35. Additionally, a beam expanding optic set 40 including first and second lenses 40A, 40B, which are generally cylindrical lenses, are added to expand light beam 15 exiting prescan lens 30. First lens 40A typically has a concave surface along the scan direction so as to diverge light beam 15. On the other hand, second lens 40B typically has a convex surface along the scan direction to re-converge light beam 15 to a sufficient width at polygon mirror 35. Scan lenses 45, 50 are then used to focus portions of the light beam 15 scanned by the polygon mirror 35 into small spot sizes on a photosensitive member 55 across all scan positions.
However, in the example approach in FIG. 1, the beam expanding optic set 40 presents added complexity and cost to the scanning system 10. Moreover, the design requiring two additional lenses for expanding beam width before the light beam reaches the polygon mirror reduces robustness of the scanning system. This is because optical performance of a scanning system is generally very sensitive to alignment of the optics before the scanning mirror. By adding two additional optical components before the scanning mirror, additional accumulated tolerances are introduced on the optical path which makes it difficult to have precise optical alignment.
Accordingly, there is a need for an improved over-filled scanner type scanning unit which is more cost efficient.